A bus system shares a common medium for connecting several units to each other. As all units are linked to one bus, communication of each terminal with each other one can be enabled at one bus system, without each unit having to be connected to each other individually. E.g. to be able to connect each one of N terminals to each other, with a star cabling layout (N−1)×N×½ connections, in particular cables, are required.
With the number of units to be connected, the number of connections, for instance cables, increases exponentially. A bus system can replace this great number of connections with a single bus to which all units are connected.
In the field of measuring and control techniques, as well as process treating systems, where sensors or actuators are linked to a controller or an analytical system, nowadays digitally communicating units are used increasingly. Herein, for intercommunication of such units, field buses, such as HART®, Profibus or Fieldbus Foundation, are implemented.
Herein, wiring of the units involved may turn out to be difficult, e.g. when obstacles, such as the course of a river or the like, have to be passed. Also, existing wiring is rather difficult to modify, and a required extension can be problematic. For complex geographical structures or widely separated measuring units wiring can eventually be complicated and expensive. Furthermore, wiring is not very flexible in case of modifications.
An Ethernet system (a very popular protocol for a LAN) is a bus system to which several parties can be connected. I.e. the units use a common medium. Special protocols of the IEEE 802.X family regulate access to the bus. With the IEEE 802.11 protocol family, as the so-called WLAN, the air was introduced as the common medium instead of a bus. All units located in the radio range of a transmitter equally receive the signals of the transmitter as if they were connected to the same bus. Thereby, wireless communication is possible in the so-called infrastructure mode or in ad hoc mode.
For wired communication by these protocols, control units or measurement evaluation units, and remote sensors or field units can communicate with each other. Sensors or field units then supply the measured values, which are evaluated by the control units. Circuitry required for wired communication is complicated and expensive and furthermore not flexible.
The international patent document WO 03/023 536 A1 provides a radio module for a field unit, which allows field units not having an integrated radio device to be operated easily and cost-effectively by radio.
Radio transmission technologies, such as WLAN or Bluetooth, make use of the so-called ISM (Industrial, Scientific and Medical Band) band for data transmission. The ISM band can be used license-free for industrial, scientific or medical applications. The 2.4 GHz-band has been released globally for industrial, scientific or medical applications. However, this band can also be used e.g. by cordless telephones or baby phones. Although conditions regarding transmitting power and interference with adjacent frequency ranges have to be met, within the free ISM band, transmission interferences of the many units operating in this band may occur. Such interferences may possibly require transmitted information to be repeated. Thus, delays can occur at the time of transmission.